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This thesis investigates the petrology and geochemistry of an abyssal peridotite dredged
from the Atlantis II Fracture Zone in the southwestern Indian Ocean Ridge. Texturally, this
sample is a serpentinized peridotite with a crosscutting coarse-grained clinopyroxenite vein.
One of the alteration veinlets contains rutile and ilmenite in association with plagioclase and
amphibole. This veinlet is not related to the pyroxenite vein. In tenns of mineralogy, the
composition of the major silicate minerals indicates that this plagioclase llierzolite represents
the depleted residue after mantle melting, similar to other abyssal peridotites from this
region.
In addition to the presence of the unique pyroxenite vein, this sample was earlier shown
to be a carrier of 'orphan Sr-87'. Unfortunately, I was unable to find such high Sr isotopic
ratios in the magnetic separations of different fractions of the sample. The sulfide
mineralogy, together with the whole rock chemistry, suggests that sea water alteration
occurs mainly as a result of serpentinization at temperatures higher than 200°C. Since the
sample is less than l Ma old, and the low temperature weathering occurred only after the
sample was exposed at the sea floor, it is possible that the weathering process was
restricted to major alteration veins. This suggests that the alteration process is highly
fracture controlled and time dependent
Trace element data from clinopyroxene grains in the peridotite shows large variations
from grain to grain. The (Ce/Yb )n ranges from 0.17 to 0.54 in the pyroxenite vein, and
from 0.75 to 2.35 away from the vein. The tendency for LREE enrichment with the
increase of distance from the vein suggests the presence of highly reacted melts. An
assimilation-fractional crystallization (AFC) model was derived which supports the idea
that the source of the clinopyroxenite vein reacts with the depleted peridotite to form a
central reaction zone. Some of the highly reacted melt, after melt-rock reaction, migrates
out of the reaction zone, and precipitates some late magmatic phases while being trapped in
the country rocks. Since the sulfide is a major Os reservoir in the abyssal peridotites, as
shown in leaching experiments, and the melt is saturated in sulfur as a consequence of the
reaction process, it is possible to model the heterogeneous distribution of the Os isotopic
data by mixing the residual peridotite with 0.2 to 0.5 wt% of sulfides precipitated from the
melt. This mixing process can explain most of the heterogeneity from 1.034 to 1.148 for
187Os/186Os. The impact on the peridotite from the melt-rock reaction and impregnation of
the late melt is obvious. As evident in Hess Deep gabbroic rocks, conductive heat loss in
the transform fault fulfils the physical requirement to create and to preserve such
geochemical signature.

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Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1997

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